Submitted to: Plant Physiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: October 14, 1997
Publication Date: N/A
Interpretive Summary: When plants freeze, ice forms outside cells in a region known as the apoplast. Substances in the apoplast which have cryoprotective properties can help protect the plant from freezing injury by causing ice to melt, by altering the size and shape of ice crystals, and by reducing the speed of ice initiation. In this study, sugars, well known cryoprotectants, were found to increase during the freezing process in the apoplast of oats to up to 17% of the level of sugar in the whole plant. Along with sugars, the storage carbohydrate fructan, a chain of fructose molecules similar to starch, was found to also increase. In addition to sugars and fructan, enzymes that are responsible for breaking down fructan into individual fructose molecules also increased. These findings will help explain how winter hardy plants can withstand freezing stresses and may provide breeders with a simple procedure to select germplasm with potential for freezing resistance.
Technical Abstract: Changes in apoplast carbohydrate concentrations and activities of carbohydrate degrading enzymes were determined in crown tissues of oat (Avena sativa, L., cv. Wintok) during cold hardening. During second phase hardening (-3 degrees C for 3 days) crown apoplast levels of sucrose, glucose and fructose increased from 0.9%, 2.1%, and 2.0%, respectively, of whole tissue levels in first phase hardened plants (3 degrees C for 3 weeks) to 6.4%, 17.6% and 14.9%, respectively. Fructan in the crown apoplast also increased from 0.5% to 0.8% in non-hardened and first phase hardened plants to 2.4% to 7.9% in second phase hardened plants. The extent of increase in apoplast fructan during second phase hardening varied with degree of fructan polymerization (e.g. DP3 and DP4 increased to a greater extent than DP7 and DP>7). Activities of invertase and fructan exohydrolase in crown apoplast increased ca. 4-fold over non-hardened and first phase hardened plants to 8.3% and 11.9%, respectively, of whole tissue activity. Apoplastic fluid extracted from non-hardened, first phase hardened and second phase hardened crown tissues had low levels (0.7, 1.1, and 2.8%, respectively) of symplastic contamination as determined by malate dehydrogenase activity. The significance of these results in relation to previously reported increases freezing tolerance from second phase hardening is discussed.